Electrical resistance and method of manufacture



Nov. 10,1936. A. H. HEYROTH ELECTRICAL RESISTANCE AND METHOD OF MANUFACTURE Filed March 29, 1932 0 2 m 0 m a. m 5 0 nv m% N .3. a O .9 r. 5. w o

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INVENTOR ALBERT .H. HEYROTH ATTORNEY Patented Nov. 10, 1936 ELEOTBICAI6FRESIST PATENT Lori-ice ANCE AND METHOD UBE l I MANUFACT Albert II. Heyroth, Niagara Falls, N. Y., aslignor to Globar Corporation Niagara Falls, N. Y., a

corporation of New York Application March 29, 1932, Serial No. 601,799 10 Claims. (01. 201-76) This invention relates to electrical resistors, and more particularly to elements of comparatively high resistance such as are used for radio resistors and for other similar purposes. The

present application is a continuation in part of my copending application, Serial No. 223,946, filed October 4. 1927.

In the manufacture of radio resistors, it is customary to use an ingredient of high electrical conductivity, as for example, carbon, and to obtain the necessary high resistance by incorporat ing therewith a material of a highly insulating character such as clay which will function both as a bonding agent and as a medium for increasing the resistance of the finished element. The resistance of the element is usually varied by varying the proportion of conducting material incorporated in the mix. The resistor mix is then extruded or otherwise formed into a rod 20 shaped element having the smallest cross-section consistent with mechanical strength. Even with a resistor of small cross-sectional area it is necessary that the conducting material constitute only a minor proportion of the mix in order to obtain the proper electrical resistance.

Owing to the large amount of insulating material employed in the manufacture of resistors of the high resistance type, the electrical properties of the-finished resistor depart rapidly from the true properties of the conducting ingredient used. For example, a carbonaceous resistor of low resistance is practically unailected either by voltage changes or by small fluctuations in temperature, and I have also found that resistors of this type do not produce microphonic noises when used for radio purposes. As the resistance is increased by the addition of clay or other similar non-conducting substances, the electrical properties undergo a marked change, especially as the high resistance range is approached; the resistor assumes the undesirable property of changing in resistance with variations in applied voltage, and also changes in resistance with only slight fluctuations in temperature such as may be produced by a loading of only one or two watts. With most resistor mixes the resistor becomes noisy or 'microphonic as the high resistance range is approached. By theterm microphonic I mean that'the resistor, when used in a circuit to in combination with a device for the conversion of electrical energy into sound, produces undesirable noises which interfere with the sounds which it is desired to reproduce. In a radio circuit involving a high degree of amplification, the microphonic characteristics of the usual radio resistor present a serious problem, since most ceramic resistors have a tendency to be'noisy, and the noise produced increases greatly with a high degree of amplification. An important characteristic of my resistors is that when they 5 are used incombination with a circuit for the reproduction of sound, the resistors are either non-microphonic or the noise characteristic is reduced to a value where it does not interfere with sound reproduction, even at relatively high 10 amplification.

I have found that the electrical properties of a resistor are dependent not only upon the character of the mix used to obtain the necessary high resistance, but also upon the specific resistance of the conducting path within the resistor, as distinguished from the total resistance desired in the finished element. If the mix is kept constant, except for the proportion of conducting ingredient, the deviation from the electrical properties desired increases rapidly as the specific resistance is increased, especially in the, high resistance ranges. I have found that this eifect can be counteracted by employing a speciflc resistance considerably less than that required if the resistor were uniform throughout its cross-section, and decreasing the cross-section of the conducting path through the resistor so as to produce a high total resistance but at the same time retain a low specific resistance in the actual portions of the material functioning as a conductor. In the processes herein described, this effect can be produced without changing the total cross-section of the element or decreasing its mechanical strength.

A resistor oi the ceramic type in which only a portion of the cross-section functions as a con,- ductor can be produced by heating the resistor under oxidizing conditions at a temperature below the vitrification or curing temperature of the ceramic material used as a bodyi'or the resistor, the heat treatment being suillciently prolonged to oxidize the conducting ingredient in the outer portions of the element, but insufiicient to affect the central portion or core. In this manner a resistor can be produced in which, for example, only one fourth to one tenth of the cross-sectional area functions as a conductor; whereas mechanical strength and total cross-section of the resistor remain unaltered. By maintaining a' low specific resistance within thev conducting path, a resistor having very desirable electrical properties can be produced even in the higher resistance ranges, and in all cases the electrical properties approach more nearly those of the con- 56 ducting material than when the conducting ingredient is dispersed throughout the entire crosssectional area of the element.

The accompanying drawing illustrates one form of my resistor and the properties attainable I in a resistor made by my process.

In the drawing: Figure 1 shows a longitudinal section of a typical resistor made by my process;

Figure 2 shows a cross-section of the resistor shown in Figure 1;

Figure 3 shows the total change in resistance of 'a 100,000 ohm resistor of one watt rated capacity up to 100 per cent overload, the change being due to both load and voltage;

Figure 4 shows the per cent permanent change of the same resistor as in Figure 2, with a loading up to 100 per cent overload; and

Figure 5 represents the change in resistance due to voltage of the resistor shown in Figures 2 and 3.

Referring to Figures 1 and 2, the shaded area 2 represents the conducting portion or core of the resistor. In this portion the material used as a conductor has not been oxidized by the preliminary treatment used to increase the total resistance. This central portion or core possesses approximately the same specific resistance as if the original mix were cured in the usual manner, and with some mixes I have actually found a decrease in the specific resistance of the core as a result of the preliminary low temperature treatment. The total resistance is, however, greatly increased. As the cross-sectional area is proportional to the square of the diameter, the area of the conducting portion decreases very rapidly as the oxidation treatment proceeds to an appreciable depth. The outer shell 3 is very nearly a non-conductor.

Electrical connection can be made with the conducting core 3 by metal spraying the end of the resistor with.a metal coating 4 of brass or other suitable metal, and capping the resistor with a metal cap as indicated by 5 or 8. Alternative to the cap, a loop of copper wire can be soldered to the sprayed portion of theresistor.

In making my resistors, I may vary the mix to suit the requirements for which the particular resistor is intended. In general, I prefer to use a mixture of a clay-like material such as bentonite, kaolin, or both, a filler, which may be either semi-conducting (as for example silicon carbide) or non-conducting (as for example, fused aluhowever, I may use a mix in which the-variableresistance properties are reduced totthe greatest possible extent even in range of high specific resistances. The properties can then be further improved by the use of a conducting core of reduced cross-section. I have found that in order to obtain the optimum properties with high specific resistances, granular non-conducting fillers such as fused alumina are very desirable. These materials, whenaddecrto the mix as arepiacement for a portion of the clay binder, effect a reduction in microphonic noises and reduce both the voltage and temperature coeflicients of resistance. In fact, the addition of fused alumina to the point where it,constitutes the greater part of the mix makes possible a resistor which will have very nearly a straight line characteristic with respect to temperature and voltage, even at comparatively high specific resistances. By rendering only a portion of the cross-section a conductor, the limits within which these characteristics are obtained are greatly extended, and the same characteristics can be obtained in a resistor of very high total resistance as would be obtained with a similar sized resistor having a very much lower resistance but in which the conducting material is dispersed uniformly throughout the cross-section of the resistor.

Typical mixes which I have found satisfactory for my resistors are as follows: (1) Low resistance mix Per cent Bentonite 20 120 grit fused alumina 20 Fused alumina powder (about 280 grit) 20 Georgia kaolin 40 The conducting material such as carbon, preferably in the form of carbon black is added to the above mix in an amount which may vary from 3 to 6 per cent, depending upon the resistance desired. The resistance of an element 1 inches long and inch in diameter can be as lowas the cross-sectional area of the conducting portion of the resistor, the mix can be used satisfactorily up to a resistance of several thousand ohms for a resist r of the above dimensions without encountering serious difficulty from variable electrical characteristics.

I have found that properties obtained in the low resistance range can be extended to resistors of much higher resistance by employing a large proportion of filler in which the particle sizes are relatively large in comparison with the particles of either the bonding agent or the carbo naceous conducting material. The efiect is one of further reducing the cross-section of the conducting path, since the finely divided conducting material is dispersed only through the interstitial material between the coarser particles, In this case also the advantage of maintaining a con ducting path of relatively low specific resistance makes possible the duplication of the characteristics of a low resistance element in the range of higher resistances. A resistor of this type may be considered as an aggregate of non-conducting or poorly conducting grains in which only the bond is an electrical conductor.

A suitable mix of the type above described is as follows: (2) High resistance mix Per cent 60 grit fused alumina 18.5 grit fused alumina--. 18.5 120 grit fused alumina 18 Fused alumina powder (approximately 280 rit) 18. Bentonite 20 Kaolin 7 Carbon black is added to this mix in amounts of from 1 to 2 per cent, depending upon the resistance desired.- By substantially reducing the cross-sectional area of the conducting portion of small diameter, the resistance of an element 1 'inchesinlengthsnd Y inchindismeteresnbs '100 ohms or less, and with proper reduction of the resistor so as to form a conducting core of increased'to considerably more than one million ohms without any seriousefiect on the electrical characteristics of the resistor. The electrical characteristics of a 100,000 ohm resistor made from theabove mix, in which the area of the fconducting core is also reduced by the oxidation treatment described, are shown in Figures 3, 4 and 5. In Figure 3, the change in resistance shown is the combined effect of temperature due to the load specified in the ordinate and the change in voltage necessary to vary the load within the specified limits. It will be noted that the change is comparatively small, even with 100 per cent overloading. With most ceramic resistors this change may amount to from 20 to 30 per cent in the 100,000 ohm resistance range, whereas in my resistor the change is only about 5 per cent.

Figures 4 and 5 show the per cent permanent change with load and the per cent change due to voltage respectively, of the same resistor shown in Figure 3. These curves very nearly approximate a straight line, whereas the change in resistance with voltage over this range is often 'as highas 30 per cent in ceramic resistors of high specific resistance, and the permanent change with load is often of considerable magnitude.

A factor which I have found of great importance in connection with the electrical properties of the resistor is the relative distribution of conducting and non-conducting material within the element. If the carbon or graphite used 'as the conductor possesses a particle size approaching or approximating that of the filler, the latter no longer functions to decrease the cross-sectional area of the conducting paths through the 'resistor, and the properties are somewhat the same as if the mix had a higher specific resistance. Furthermore, the more discontinuous the conducting paths through the resistor, the greater is the difficulty from 'microphonic noises.

Both kaolin and bentonite are more or less col- 'rial and over the surface of the particles of filler so as to produce a resistor in which the properties of a conducting path of low specific resistance are retained even when the total resistance is comparatively high. I have found that a ceramic resistor of the type described possesses better electrical characteristics and is less subject to the production of microphonic noises than is a similar resistor made from the various forms of powdered graphite or powdered carbon in which the state of subdivision is not the same order of magnitude as that obtained with carbon black.

If silicon carbide is used as a filler, the following mix is suitable:

1 Per cent Silicon carbide; 70 Bentonite 20 Carbon black 5 ried .over wide limits, depending upon the particular type of resistor desired. The fillers need not be confined to the material specified, but flint, zircon or other non-conducting materials having a fusing point higher than the vitrification temperature of the clay used as a bond can also be used. Fused alumina, however, owing to its highly infusible nature and its comparatively slight tendency to react with the clay. bond or to dissolve therein, possesses a number of advantages. The clay-like substance used as a bond can also be varied both in character and quantity.v ,I have found, however, that bentonite facilitates the distribution of the carbon through the mix, and also aifords a high green strength and a high degree of toughness to the unburned resistor. .Bentonite also imparts a slipperiness to the mix which is advantageous in the case of extrusion.

In carrying out my process, the raw ingredients are intimately mixed and sufiicient water is added to the mix to impart plasticity. The resisters can then be extruded or otherwise formed into rods and dried in the presence of air at a temperature of, for example, 200 to 300 F.

After the resistors are dried they are given the preliminary oxidation treatment to oxidize the conducting material in the outer portions of the resistor. The temperature used for this treatment is preferably a dull red heat, as for example 1200 F., although the carbon will oxidize appreciably over a prolonged period of time at temperatures as low as 900 F. Higher temperatures can also be used, providing'the incipient fusion or vitrification point of the clay binder is not exceeded. With most mixes the oxidizing temperature should not exceed approximately 1900 F. The time necessary'will depend upon the mix, the temperature used and the depth to which oxidation is desired, and must be determined by trial. .The treatment is carried out in an oxidizing atmosphere, preferably in an electric furnace. With a given mix and temperature, resistances can be accurately duplicated providing the furnace conditions are kept comparatively constant.

' Although the resistors may harden to a certain extent during the preliminary oxidation treatment, especially if the latter operation is carried out at a comparatively high temperature, it is desirable to subject the resistors to a subsequent firing operation at a higher temperature in order to effect at least a partial vitrification of the clay binder or a sintering of the resistor into a strong coherent mass. With the bentcnitekaolin-fused alumina mixes above specified, a

found to make a diiference in resistance of approximately 200 times with certain mixes, whereas the commercial tolerance limits are usually only 10 per cent.

, Other conducting materials than carbon can be used in making resistors in accordance with my process. For example, easily oxidizable metfinal firing temperature of from 2200 to 2400 F.

- in comparison als such as aluminum or magnesium can be incorporated into the mix and the outer portion or shell of the resistor oxidized as described. Car- There are also many applications where a conductor of small diameter ofiers special advantages, as for example in a resistor subjected to a current having a frequency of the order of magnitude of radio frequencies. With a high frequency current the conduction is largely on the surface of the conducting medium, so that the diameter becomes an important factor. Resistors in which the total diameter is extremely small are impractical from the standpoint of mechanical strength, but with my process the strength is in no way dependent upon the crosssectional area of the conducting portion.

I claim: Y

1. A radio resistor characterized by non-microphonic properties, the said resistor comprising non-conducting refractory grains, a clay-like binder and finely divided carbon particles dispersed within the said binder, the said refractory grains being many times the size of the said carbon particles and also many times the size of the particles of clay making up the binder, whereby the resistor is characterized by paths of comparatively low specific resistance in the interstitial spaces between the said refractory grains.

2. The method of making a non-metallic radio resistor of high electrical resistance which comprises forming a mix containing carbon, a clay binder and a filler of poor electrical conductivity with carbon, forming the resistor, and heating the resistor in an oxidizing atmosphere at a temperature below the vitrification point of the binder to form an outer non-conducting shell of substantial thickness in comparison with the diameter of the resistor, leaving a core of high specific conductivity in comparison with the said outer shell, and subsequently firing the resistor to at least partially vitrify the clay binder.

3. The steps in the process of making a radio resistor which comprise forming the resistor from a mix containing an oxidizable conducting ingredient and a, clay-like binder, heating the iormed resistor in an oxidizing atmosphere at a temperature sufllcient to oxidize the conducting ingredient but insufficient to vitriiy the clay-like binder until the conducting ingredient is oxidized from the outer portion of the resistor and remains only within a core of substantially reduced crosssectional area in comparison with the total cross sectional area of the resistor, and thereafter curing the resistor at a higher temperature than that used in the oxidation treatment, the said higher temperature being sufficient to at least partially vitrify the clay binder.

4. .The steps in the process of making a radio resistor which comprise forming the resistor from a mix containing carbon, clay and a plurality oi refractory grains many times the size of the particles of carbon and clay, heating the formed resistor in an oxidizing atmosphere at a temperature sufiicient to oxidize the carbon but insufficient to vitrify the clay until the carbon is oxidized from the outer portion of the resistor and remains only within a core of substantially reduced cross-sectional area in comparison with the total cross-sectional area of the resistor, and thereafter curing the resistor at a higher temperature than that used in the oxidation treatment, the said higher temperature being sufficient to at least partially vitrify the clay binder.

5. A radio resistor consisting of a self-sustaining rod of high electrical resistance, the said rod being an aggregate of, crystalline refractory grains bonded with a clay-like binder, a minor proportion of a carbonaceous conducting ingredient being distributed through a portion only of the said hinder, the resistor having an inner conducting core of substantially reduced cross section in comparison with the cross section of the resistor and an outer non-conducting shell of substantial thickness in comparison with the diameter of the resistor, the said shell being integral and continuous with the core and having the same composition as the core except for the conducting ingredient, the crystalline refractory grains within the rod being many times the size of the particles of carbon and clay making up the binder.

6. A rod-like resistor element for electrical circuits, comprising a resistance core ofelectrical conducting material distributed in a vitrified insulating matrix, and a rigid ceramic jacket surrounding the core and vitrified into an integral mass with said core.

'7. A rod-like resistor element for electrical circuits, comprising a resistance core of electrical conducting material distributed in a ceramic binder, and a ceramic jacket surrounding lire core, the conducting material and the binder being vitrified into a stable mass in intimate contact with the ceramic jacket.

8. A rod-like resistor element for electrical circuits. comprising a resistance core of electrical conducting material of high specific resistance, a ceramic binder for said material, and a ceramic jacket surrounding the core, the conducting material and the binder of the core being vitrified into a stable mass in intimate Contact with the ceramic jacket.

9. A rod-like resistor element for electrical circuits, comprising a resistance core of electrical conducting material distributed in a heat hardened ceramic insulating matrix, and a heat 3 hardened rigid ceramic jacket surrounding and protecting the core and in intimate contact with the core, the element being fired so as to effect at least partial vitrification oi the ceramic ma-' supporting the core, the element being heat hardened to consolidate it into a stable mass with the core and ceramic jacket in intimate contact.

ALBERT H. HEYRO'I'H. 

